Remove duplicate relative url in package registry

What does this MR do and why?

This MR aims to solve a long standing bug mentioned in issue #332884 (closed) .

It removes method call to #expose_path as it leads to adding duplicated relative url part inside "Package Registry > Registry setup" URL.

Files changed:

/app/helpers/packages_helper.rb

How to set up and validate locally

1. Setup GDK environment locally & clone gitlab.
2. Run bundle exec rails console in the gitlab directory to use gitlab in rails console.
3. Follow through initial steps mentioned in this comment & make a function call to #package_registry_project_url in above MR branch.

MR acceptance checklist

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